1. Field of the Invention
This invention relates to systems and methods for sensing any of position, displacement, velocity, acceleration, area, and volume. Such systems and methods may be useful in industrial automation, microscopy, surface profiling, scanning, printing, material deposition, and metrology.
2. Description of the Related Art
Linear encoders are used to measure variable linear positions of industrial machinery and generate output signals of these positions, rather than simply indicate the presence or absence of a target with discrete on-off signals, as do proximity sensors. The encoders are essential for precise operation of machines in fabrication or robotics applications.
A typical optical encoder consists of two parts: a scanning unit and a scale. The scanning unit contains a radiation source, a condenser lens, a reticle with grated windows, and a photodetector. Most manufacturers use line-grated glass or metal scales that mount on a machine base, while the unit is connected to the moving slide of the machine. When the scanning unit moves, a parallel beam of light passes through the condenser lens, then through the windows on the scanning reticle, and onto the reflective grated scale. Reflected light passes back through the scanning windows and strikes the photodetectors. These sensors convert the fluctuation in light intensity into analog sinusoidal electrical signals that are phase shifted by 90.degree. These outputs are sent to a digital readout or numerical controller for interpolation and subsequent decoding to give an up/down count showing the position of the moving slide. There are two types of linear encoders—incremental and absolute. An incremental encoder needs to find the home position (origin) every time it is turned on. It then calculates the position by measuring incremental distance from home. An absolute encoder can determine its position after being turned on without homing operation. In a conventional optical encoder, the absolute measurement is achieved by using a complex grating that indicates absolute position information in addition to incremental scale divisions.
Conventional grating-based encoders suffer from various limitations that restrict their utility. One limitation is the high cost of calibrated gratings. This cost is elevated due to the necessities of high precision fabrication, the use of stable (e.g., thermally stable) materials, and the functional relationship between cost and length (since longer encoders require commensurately longer gratings). Currently, encoders cost several hundred dollars per meter per axis in mass quantities. It is common for a multi-axis industrial machine or robotic apparatus to use multiple encoders, with one encoder for each degree of freedom of movement. Absolute encoders are typically more expensive than incremental ones due to the increased complexity. Another limitation associated with conventional linear encoders is their sensitivity to scratches, damage, and contamination. Yet another limitation associated with conventional linear encoders is their limited ability to provide extremely fine resolution, particularly without substantially increased cost.
Any optical lens or system has a viewing angle that results in undesirable scale change relative to the object distance from the lens. Telecentric lenses, which provide depth of field while holding magnification constant, have been developed to minimize this effect. A typical commercial telecentric lens has a viewing angle of 0.2 degree. Such a viewing angle, while small, still causes a perceptible scale change that limits the measuring accuracy and affect the mounting tolerances for an optical system.
One encoder that addresses certain limitations associated with conventional encoders is disclosed in U.S. Pat. No. 6,246,050 to Tullis, et al. (“Tullis”). Tullis discloses an optical encoder having a photosensor array that detects relative movement of an uncalibrated target surface (e.g., a surface having natural surface features). The photosensor array generates a sequence of data frames of the imaged areas, and a processor processes patterns in the data frames to detect a relative motion or displacement of the target to determine incremental relative motion or rate of relative motion. To enhance detectability of some random surface features, the target surface can be illuminated at a high incidence angle (e.g., 15 to 60 degrees) relative to the surface normal. A telecentric lens may be used between the target surface and photosensor (Tullis, col. 9). For purposes of absolute position measurement (described as useful to eliminate runout errors in otherwise incremental position measurements), Tullis (at col. 10) further discloses the addition of a unique and identifiable pattern, such as (1) a printed density that varies as a sine wave with continuously increasing spatial frequency, or (2) a pair of diverging lines, overlaid on top of a random field. Images of these printed patterns are compared with previously captured patterns or reference images to output pulses when correlations are found. Tullis teaches that “absolute measurement is thereby made when the second correlation is found” (Tullis, col. 10, lines 39-41), suggesting some calibration between printed pattern placement and absolute position. In this regard, Tullis's utilization of a printed pattern is analogous to the use of a calibrated scale, with the inherent drawbacks attendant to such a scale.
Tullis suffers from certain limitations that restrict its utility. A device according to Tullis may have insufficient speed at high-resolution operation to be suited for use with target surfaces of extended length. The unique and identifiable patterns taught by Tullis for providing absolute position measurement may also have limited application to high-resolution target surfaces of extended length. Furthermore, Tullis fails to address potential problems associated with surface alteration. Additionally, Tullis describes the use of telecentric lenses (which limit measuring accuracy).
Based on the foregoing, there is a need for improved systems for positional sensing systems. Ideally, improved systems would be economical, versatile, and adapted to provide extremely fine positional sensing resolution.